The present invention relates to a device for discriminating fluids of different refractive indices and to a device for measuring the voluminal fraction of at least one fluid of a current of non-mixable fluids comprising the application of the discriminating device. The term "current" used in this description means a relative movement of the fluids of the current relative to the detector of a discriminating or measuring device.
The principle of operation of such a fluid discriminating device is based on the difference between the indices of refraction, for example between a gas or a vacuum (equal to 1) and a liquid (in respect of water 1.3). Indeed, when a beam of light propagated in a multimode optical fibre reaches its end, the coefficient of reflection undergone by the beam depends on the difference between the refraction indices between the fibre and the outer medium in contact with the end of the fibre. Thus, by measuring the returning light, it is possible to deduce therefrom at any moment the index of refraction of the fluid in contact with the end of the fibre, and therefore the nature of the fluid (gas or liquid). By effecting a mean with respect to time, there is obtained the local voluminal fraction of each fluid in the current in the region of the end of the probe and it is then possible to obtain the voluminal fraction by extension to a certain volume around the end of the fibre as a function of the chosen hydraulic model.
Known devices performing such a function comprise a gas laser for example of the helium-neon type, whose beam is coupled in a multimode optical fibre by optical coupling means (lens) after having passed through a separating strip, this fibre being used directly as a measuring probe. The reflected beam is received by a photomultiplier. The use of a photomultiplier rendered necessary by optical losses of the chain, and its association with a gas laser and optical coupling elements, require mechanical assemblies and result in a large and relatively fragile device. Moreover, owing to the arrangement using a single fibre, the measuring probe, subjected to severe surrounding stresses, can only be replaced by completely disassembling the device. Lastly, as the laser is a continuous laser, the return light cannot be distinguished from the surrounding light without the use of an additional costly, fragile, large or limiting device and it is therefore necessary to maintain this surrounding light at a sufficiently low level.